The ultimate level of light confinement has been reached.
Researchers at ICFO - The Institute of Photonic Sciences in Barcelona, along with other members of the European research initiative Graphene Flagship, have confined light down to a space of one atom. The work could pave the way to optical switches, detectors and sensors with a thickness of a single nanometer.
Because the applications of light include ultra-fast communications, ultra-sensitive sensors and on-chip nanoscale lasers, shrinking the devices to control and guide light has been the subject of significant research. Previous techniques had found that using metals to compress light below the wavelength scale (diffraction limit) would inevitably involve energy loss. The new research on graphene, however, represents a breakthrough.
"Graphene keeps surprising us: Nobody thought that confining light to the one-atom limit would be possible,” says Prof. Frank Koppens of the Catalan Institution for Research and Advanced Studies (ICREA), who led the research. “It will open a completely new set of applications.”
The researchers chose graphene because of its ability to guide light through electron oscillations, also known as plasmons. Stacks of two-dimensional materials were used to build up a new nano-optical device: a graphene monolayer (which acts as a semi-metal) was stacked onto it a hexagonal boron nitride (hBN) monolayer (an insulator); on top of this was deposited an array of metallic rods.
"At first we were looking for a new way to excite graphene plasmons. On the way, we found that the confinement was stronger than before and the additional losses minimal. So we decided to go to the one-atom limit with surprising results," says lead author David Alcaraz Iranzo of ICFO.
By sending infra-red light through their devices, the researchers observed plasmon propagation between the metal and the graphene. Minimizing their light confinement space by reducing the gap between metal and graphene did not show a loss of efficiency; even when a monolayer of hBN was used as a spacer, the plasmons could still propagate freely. By simply applying an electrical voltage, the researchers were able to switch plasmon propagation on and off — demonstrating the control of light within channels smaller than one nanometer.
The research represents a new kind of exploration of extreme light-matter interactions, as well as a promising application of the atom-scale toolbox of two-dimensional materials. The work has been published in a recent edition of the journal Science.